The invention relates to a lifting device for a working station of a packaging machine, the working station having a first tool member and a second tool member mounted for movement relative to the first tool member.
A packaging machine conventionally comprises working stations for various operations such as forming, filling, sealing and cutting. A lifting device moves at least a movable member of a working station into its operating position. In this position an enhanced closing force for closing the tool members of the working station is required to lock a packaging web for carrying out the operation. For example, a chamber is hermetically closed for molding a portion of a plastic sheet material and thereafter the sheet material is molded using fluid from a pressurized air source. The high pressures used for a fast and exact molding operation put a high load on the forming station.
In order to receive this high load in the closed state of the working station it is necessary to have closing members producing a closing force which is greater than the load imposed during the operational phase. Simple members consist of piston-cylinder devices having a correspondingly large effective area. However, such members are very expensive because of their high air consumption for the entire lifting stroke. Other members consist of toggle lever systems producing higher forces at the end of the travel by mechanical transmission. It is a drawback of such toggle lever systems that they produce a defined closing movement having a so-called dead point. Thus, close tolerances must be observed on the one hand for generating the required closing force at the dead point and on the other hand the forces are varying dependent on the thickness of the web material. However, the thickness of the material depends on the intended use and may therefore vary within wide limits. Hence, the adjustment of the lifting device must be adapted after each change of the thickness of the material. This demands a lot of the operator.
German patent application 42 16 207 discloses a lifting device for a cutting station of a packaging machine. The lifting device comprises a piston-cylinder device with a piston rod being adapted to form an abutment at its free end. At the end of a lifting travel produced by the piston-cylinder device the piston rod is further lifted by pushing a wedge-shaped member into a corresponding recess of the piston rod to produce, in the final position thus obtained, a force which is higher than the force which can be produced by the piston-cylinder device. However, the friction forces occurring by pushing in the wedge cause a rapid wear of material.